Aspects of the present disclosure and embodiments thereof include methods and systems for providing security for a document. For instance, security is provided by varying or modulating the optical density of a portion of a document having a level of optical density at which the human visual system (HVS) has difficulty perceiving variations in optical density or at an optical density modulation or variation amplitude at which the HVS has difficulty perceiving. The variations or modulations in optical density may be provided by overprinting. Alternatively, modulation may be achieved by modifying image data. Embodiments are described with reference to a text image and a text watermark. However, the methods and systems disclosed herein can be applied to other images and may apply to other watermarks, including, but not limited to, bar codes and data glyphs.
Typically, watermarking and other security features are applied to high value or high volume documents. For example, security features are included in currency, checks and negotiable securities. The protection of high volume and high value documents can justify relatively complicated security mechanisms. Additionally, high value and high volume documents can justify relatively complicated document design. For instance, time can be spent designing visually pleasing tints or halftoned images that can be used to mask or otherwise accommodate watermarking techniques.
For example, U.S. Pat. No. 6,721,440 B2 to Reed, et al., issued Apr. 13, 2004 for Low Visibility Watermarks Using an Out-of-Phase Color discusses embedding a media signal with a digital watermark component. The media signal includes a cyan color plane, a magenta color plane, a yellow color plane color and a black plane. The digital watermark component is embedded in cyan, magenta and yellow color planes. The digital watermark component is inverted and embedded in the black color plane. The protection of high volume and high value documents may justify the inclusion of cyan, magenta, yellow and black color planes or separations that can accommodate such security schemes.
However, there has been desire for systems and methods for protecting or watermarking less complicated, low volume, low value documents such as, for example, coupons and gift certificates that might be produced and distributed for or by a small business or organization.